WallyWorld won't have what you need (although... I'll get to that in a minute).
If you want a thermostat, which I think is better, then the least expensive route is to get a 500R (also a Zoo-med product). At least for me, they're harder to find locally since PetBox (can't remember it was 'Co' or 'Smart') stopped carrying them.
You can also go online and find it, and other more 'high end' units. Google The Bean Farm and see what they have. The HerpStat progressive units are very good and easy to use.
Back to Wal*Mart for a second. Instead of a thermostat, which regulates based on a temperature sensing, you could use a rheostat, which is a fancy way to say "dimmer switch." It isn't all that hard to make your own using an extension cord, dimmer switch and an electrical box you can get at Wal*Mart for under $10 and have enough change for a burger. The big difference between a dimmer and a thermostat, is that as the temperature changes, you have to adjust the dimmer up and down to keep the UTH at the temperature you want. That's essentially what a thermostat does if you think about it.
Either way is FAR better than letting a UTH run unregulated. It'll just get too hot for your snake without it.